(a) Field of the Invention
This invention relates to a method of geophysical exploration employing liquid-filled geophones, which do not require orienting and which are especially adapted for being towed by a flexible member over the earth's surface. The method is characterized in that the electric signals produced by the geophones have a polarity which is independent of the orientation of the axes of the geophones relative to the vertical over an arcuate span ranging from 0.degree. to 180.degree..
(b) Reference to Related Prior Art
In reflection seismic prospecting, the reflected seismic signals are detected with seismic detectors, either geophones or hydrophones. Geophones are typically for land use and hydrophones for marine use. In use, a geophone is coupled to the earth and is responsive to movements thereof. A hydrophone is submerged in water and is responsive to the pressure changes produced therein by the movements of the earth. Either type may be used in shallow waters, typically positioned on the bottom of the body of water. For the sake of simplicity, the term geophone as used herein shall be understood to also include seismic detectors used on or in water. Most practical geophones presently commercially employed constitute an electromagnetic circuit which includes a coil and a magnet movable relative to each other. A conventional geophone is substantially responsive only to movements of the earth having a component force along its axis and, in use, produces an output electric signal having a polarity dependent upon the direction of such force.
During normal seismic prospecting, geophones are manually disposed upright on the earth's surface with their axes substantially in alignment with the vertical. When a geophone accidentally or otherwise becomes disposed on the earth's surface such that its axis is excessively inclined from vertical, or in the extreme case, when the geophone is completely on its side (90.degree. from vertical), it becomes inoperative for all practical purposes. Also, when a geophone becomes accidentally disposed upside-down (180.degree. from vertical), it will produce a signal having a polarity which is opposite from the polarity of the signal produced by the geophone when upright. In either case, the failure to produce an output signal, or the production of an opposite polarity signal, adversely affects the records produced by the seismic prospecting.
To overcome the problems when such upright positioning is impractical, the geophones are mounted on orienting means which are adapted to continuously maintain the geophones in an upright position. A very common orienting means is provided by a gimbal mount. More recently, attempts have been made to mount the geophones on a flat, flexible, conveyor-type belt which is towed by a vehicle. The belt is intended to maintain the geophones substantially upright. The dragging of such belts in many instances has not produced the anticipated results, and, therefore, seismic crews still continue to spread out the geophones manually in an upright position, all of which is time consuming and very expensive.
As the search for hydrocarbons is increasingly expanding to relatively inaccessible terrains, the need to uprightly dispose the geophones over the earth's surface, or to use self-orienting means, such as gimbal mounts, places a heavy burden on the seismic crews. This burden has plagued the seismic industry for many years in spite of continuous efforst to devise geophones which can be towed and which need not be uprightly implanted on the earth's surface, as above described.
After long experimentation I have devised a novel, liquid-filled geophone which can effectively replace conventional geophones, especially of the electro-magnetic type that have been almost exclusively used commercially to date.
Accelerometers using liquids as described in U.S. Pat. Nos. 3,270,565, 3,555,543 and in U.S.S.R. Inventor's Certificate 171,676 issued on July 26, 1965. Such liquid-filled devices, however, produce a response having the same polarity for all excitations, irrespective of direction, or are incapable of meeting the peculiar requirements imposed by seismic prospecting.
The novel geophone is relatively light weight, inexpensive to manufacture, capable of withstanding normal field abuse, and, above all, need not be implanted on the earth's surface in any particular orientation. The new and improved geophone therefore requires no self-orienting means which are expensive and frequently fail during normal field abuse. The polarity of the output signal produced by this novel geophone can be made to be independent of the inclination of the geophone's axis relative to the vertical. For upwardly and downwardly directed movements of the earth, this geophone will produce electric signals of opposite polarities, as required for seismic prospecting.